Cellulosic structure and method for preparing same



' Patented Oct. 19, 1937 CELLULOSIC STRUCTURE FOR PREPARING SAME AND- METHOD Dorothy Bateman Maney, Old Hickory, Tenn, assignor, by mesne assignments, to E. L du Pont de Nemours & Company, Wilmington,

DeL, a corporation of Delaware No Drawing. Application November 6, 1934, Serial No. 751,748

17 Claim.

This invention relates to coated cellulosic materials and more particularly it relates to a nonfibrous, cellulosic sheet or film having a surface coating securely anchored thereto, and a method 5 of anchoring a surface coating to such a cellulosic sheet or film.

Cellulosic sheets or films provided' with a sur face coating, such as a moistureproof surface coating, have recently been developed and are very extensively used as wrapping material for various articles of manufacture.

Such known coated cellulosic materials are particularly adaptable as coverings for articles of food, for example, candy, biscuits, coffee, and the like, which articles have a comparatively small moisture content, for the purpose of maintaining the article substantially at its original moisture content. Such known coated cellulosic materials are unsatisfactory as a wrapping material for foodstuffs having a high moisture content, for example, ice cream, butter, fresh meat and the like where the wrapper is in direct contact with such high moisture containing articles. When such material isused in direct contact with articles having a high moisture content, the coating separates and/or flakes ofi from the base sheet in a comparatively short period of time with the result that the material becomes soggy, presents a very unsightly appearance and is of substantially no value as a wrapping material.

It has now been found that a surface coating may be firmly anchored to the base sheet or film by providing an extremely thin layer of sulfur between the base sheet and the surface coating. 5 It is therefore an object of this invention to provide a method for securely anchoring a surface coating to a cellulosic, non-fibrous base It is a still further object of this invention to provide a cellulosic sheet or film having a moistureprooflng coating thereon and a thin layer of sulfm' between said coating and base sheet.

Other objects of this invention will appear hereinafter.

55 The details 91 this invention will be set forth with specific reference to base sheets of re erated cellulose, it being understood that such regenerated cellulose base sheets can be substituted by any non-fibrous, cellulosic sheet or film precipitated from an aqueous, alkaline, cellulosic 5 dispersion, such as glycol cellulose, cellulose glycolic acid, methyl cellulose, cellulose phthalic acid, and the like. The process may also be applied to a base consisting of a cellulose ester or ether precipitated from solution in organic solvents, 10 such as cellulose acetate, cellulose propionate, benzyl cellulose, and the like.

According to the preferred practice of this invention, a finished, dry and softened sheet or film of regenerated cellulose, made in the usual man 15 ner on regular plant equipment for the manufacture of regenerated cellulose sheeting, is ex posed to the vapors of sulfur monochloride for a brief interval of from 1 to 5 minutes. An extremely thin coating of free sulfur is formed by the sulfur monochloride vapors coming in contact with the surface of the film and is deposited uniformly on the surface thereof. This deposit of sulfur on the surface of the film .is so minute as to be invisible to the naked eye and has substantially no effect on the transparency of the film. The film is subsequently coated with'a moistureproof lacquer comprising, for example, a cellulose derivative, a blending agent, a wax and a plasticizer such as is well known in the art. The surface coating of the resulting sheet or film is found to be securely anchored to the base sheet and will remain securely anchored thereto, even when placed in direct contact with water or materials having a comparatively high moisture con- .tent, for a considerable period of time.

As coating compositions which can be securely anchored to a cellulosic base by means of a minute layer of free sulfur, this invention eontemplates any film-forming composition containing a cellulose derivative base, such as nitrocellulose, cellulose acetate, ethyl cellulose, methyl cellulose, or other similar cellulose esters or ethers, or a composition containing an ether-resin base, by which is meant a resin in the formation of which ether linkages are formed, and which may be produced by the reaction of a polyhydric phenol with an aliphatic polyhalide, or a com- Anyvolatile sulfur compound which can be readily decomposed to yield free sulfur, such as, for example, thionyl chloride, sulfonyl chloride, thionyl bromide, and the like, all of which compounds decompose spontaneously in the presence of moisture, may be'substituted for sulfur'monochloride as a means for securinga deposit of free sulfur on the surface of the cellulosic base. A deposit of free sulfur on the surface of the cellulosic base can also be secured by the use of aqueous solutions of sulfur compounds which require acids or oxidizing agents to liberate free sulfur. Thus, a cellulosic base, for example, gel regenerated cellulose, might be passed through an aqueous solution of sodium thiosulfate, potassium thiosulfate or ammonium thiosulfate and subsequently exposed to a current of air containing small quantitles of acid vapors, or to a current of some weakly acid gas to liberate free sulfur from the thiosulfate. Likewise, a cellulosic base might be passed through a dilute solution of a weak oxidizin agent, for example, a dilute aqueous solution of ammonium sulfite might be applied to a cellulosic base and the resulting film led immediately through hydrogen sulfide vapors to secure a deposit of free sulfur on the cellulosic base.

The base sheet, prior to the application of the anchoring coating of sulfur, is preferably in the finished, dried and softened form; nevertheless sheets which have been merely bleached and washed and are still in the gel condition may be coated with sulfur and dried prior to application of the surface coating to secure satisfactory anchoring between the surface coating and the base sheet. In this latter case, the sulfur is preferably formed on the base sheet from a solution of a suitable sulfur compound, which solution also contains an amount of glycerin or other softening medium, sufilcient to suitably soften the base sheet.

In order more clearly to set forth the practice in accordance with this invention, several specific, illustrative examples are hereinafter set forth, it being understood that theseexamples are not intended to restrict the scope of the invention.

Example I.Regenerated cellulose film which has been cast, bleached, washed, passed through a glycerol or other softening bath and dried in the usual manner, is exposed to the vapors of-sulfur monochloride by passing it for a period of preferably l to 5 minutes through a chamber at the bottom of which is liquid sulfur monochloride and through which air is slowly circulated. After acquiring a deposit of free sulfur from exposure to the decomposing sulfur monochloride vapors,

"the cellulose film may, if desired, be subjected to a current of air or dilute ammonia vapor to remove from the film any trace of hydrogen'chloride or sulfur dioxide formed during the decomposition of the snlfur monochloride.

The film is then passed through a moistureproof coating composition comprising:

- Percent Percent Nitrocellulose 6.44 Ethyl acetate"--- 55.85 Gum damar 2.17 Toluene 28.70 Dibutyl phthalate. 2.40 Ethanol 3.59 Paraffin 0.52 Acetone 0.15 Zinc stearate 0.17 Benzene 001 The excess solution is 'removed'by suitable means, such as glass rods or heated doctor knives, and the coated film is dried at a temperature above the melting point of the wax in the composition, after which it may be humidified by passing the same through an atmosphere havin8 an elevated temperature and high humidity. The finished coated cellulosic film retains its surface coating tenaciously in direct contact with water for days or even weeks (3% to 15 days) and is very satisfactory when used as a wrapping directly in contact with products containing large amounts of water or moisture such as butter, cheese, wet or frozen fish, ice cream, and the like, for prolonged periods of time. I

Example II.Regenerated cellulose film which has been cast, bleached and washed in the usual manner but has not been dried and is still in the gel condition is passed through an aqueous solution containing 4%% glycerol and 5% ammonium sulfite. The film is then passed through squeeze rolls to reduce the amount of solution present in the film to approximately 3% times the weight of the cellulose and is next dried at elevated temperatures in an atmosphere ofhydrogen sulfide. During the drying operation, the ammonium sulfite in the film is reduced by the hydrogen sulfide resulting in the deposition of free sulfur on the cellulosic film.

The film is then humidified if desired and thereafter coated with a moistureproofing composition comprising essentially a cellulose derivative base, a wax or wax-like material, a plasticizer and a blending agent, and dried at temperatures above the melting point of the wax or wax-like material present in the coating.

For the purposes of this specification and claims, we define moistureproof materials as those which, in the'form of continuous, unbroken sheets or films, permit the passage of not more than 690 grams of water vapor per 100 square meters per hour, over a period of 24' hours, at approximately 39.5 C. plus or minus 0.5 C., the relative humidity of the atmosphere at one side of the film being maintained at least at 98% and the relative humidity of the atmosphere at the other side being maintained at such a value as to give a humidity differential of at least Moistureproofin'g coating compositions are defined as those which, when laid down in the form of a thin, continuous, unbroken film applied uniformly as a coating with a total coating thickness not exceeding 0.0005" to both sides of a sheet of regenerated cellulose of thickness approximately 0.0009", will produce a coated product which is moistureproof.

For the purposes of experimental tests, especially for those -materials adaptable as coatin compositions, moistureproof materials include those substances, compounds or compositions which, when laid down in the form of a continuous, unbroken film applied uniformly as a coating with a total coating thickness not exceeding 0.0005" to both sides of a sheet of regenerated cellulose of thickness approximately 0.0009", will produce a coated sheet which will permit the passage therethrough of not more than 690 grams of water vapor per 100 square meters per hour over'a period of approximately 24 hours, at a temperature'of 895 C. plus or minus 0.5" C. (preferably 395 C. plus or minus 0.25 C.), with a water vapor pressure differential of 50-55 mm. (preferably 53.4 plus or minus 0.7 mm.) of mercury. For convenience, the number of grams of water vapor passed under these conditons may be referred to as "the permeability value. An uncoated sheet of regenerated cellulose having a thickness of approximately 0.00097 will show a permeability value of the order of 6900.

Non-fibrous, transparent, cellulosic sheets ortureproof surface coating, securely anchored thereto, may be produced in accordance with this.

templated by this invention will not noticeably aifect the transparency and luster of the base sheet or surface coating, will not give off any noticeable odor, and is non-toxic.

Obviously, various changes and modifications may be made in the above detailed description without departing from the nature andspirit thereof. It is therefore to be understood that the invention is not to be limited to the specific modifications except as set forth in the appended claims.

I claim:

1. The method of making a sheet or film suitable for use as a wrapping material which comprises passing a base sheet of non-fibrous, transparent, cellulosic material into contact with vapors of sulfur monochloride and subsequently providing said sheet with a surface coating, said surface coating being applied over and in contact with the layer of sulfur formed on the base sheet.

2. The method of making a sheet or film suitable for use as a wrapping material which comprises passing a base sheet of non-fibrous, transparent, cellulosic material into contact with vapors of sulfur monochloride for a period of from 1 to 5 minutes, and subsequently providing said sheet with a surface coating, said surface coating being applied over and in contact with the layer of sulfur formed on the base sheet.

3. The method of making a sheet or film suitable for use as a wrapping material which comprises passing a base sheet of non-fibrous, transparent, cellulosic material through an aqueous solution containing ammonium sulfite, contacting said sheet with hydrogen sulfide, and subsequently providing said sheet with a surface coating, said surface coating being applied over and in contact with the layer of sulfur formed on the base sheet. 4

4. The method of making a sheet or film suitable for use as a wrapping material which comprises passing a base sheet of non-fibrous, transparent,-cellulosic material through an aqueous solution containing ammonium sulfite, drying said sheet in the presence of hydrogen sulfide, and subsequently providing said sheet with a surface coating, said surface coating being applied over and in contact with the layer of sulfur formed on the base sheet.

5. The method of making a sheet or film suitable for use as a wrapping material which comprises forming on a non-fiborus base sheet of cellulosic material a minute, substantially transparent layer of sulfur, and subsequently coating said base sheet with a film-forming composition, such composition being applied over and in contact with said layer of sulfur, whereby to obtain an adhesion between said sheet and the film formed from said composition greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with each other, said adhesion being tenacious even when 'the, composite film is subjected to contact with water over a long period of time.

. a long period of time.

sheet of non-fibrous transparent cellulosic material, said layer being so minute as to have substantiallyno effect on the transparency of the 1 sheet, and subsequently coating said base sheet with a film-forming composition, said composi tion being applied over and in contact with said layer of sulfur, whereby to obtainan adhesion between said' sheet and the film formed from said composition greatly improved over that exhibited between a similar sheet and surface coating when in. direct contact with each other, said adhesion. being tenacious even when the composite film is subjected to contact with water over a long period of time.

7. The method of making a sheet or film suitable for use as a wrapping material which comprises forming on a base sheet of regenerated cellulose a minute, substantially transparent layer of sulfur, and subsequently coating said basesheet with a film-forming composition, such composition being applied over and in contact with said layer of sulfur, whereby to obtain an adhesion between said sheet and the film formed from said composition greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with each other, said adhesion being tenacious even when the composite film is subjected to contact with water over 8. The method of making a sheet or film suit able for use as a wrapping material which comprises forming a thin layer of sulfur on a base sheet of non-fibrous transparent cellulosic material, said layer being so minute as to have substantially no effect on the transparency of the sheet, and, subsequently applying to said base sheet a surface coating composed of a film-forming base and a waxy material, said surface coating being applied over and in contact with the layer of sulfur formed on the base sheet whereby to obtain an adhesion between said sheet and the film formed from said surface coating greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with each other, said adhesion being tenacious even when the composite film is subjected to contact with water over a long period of time.

9. The method of making a sheet or film suitable for use as a wrapping material which comprises forminga thin layer of sulfur on a base sheet of non-fibrous transparent cellulosic material, said layer being so minute as to have substantially no effect on the transparency of the sheet, and subsequently applying to said base sheet a surface coating composed of a film-forming base, a waxy material and a blending agent,

said surface coating being applied over and in contact with the layer of sulfur formed on the base sheet whereby to obtain an adhesion between said sheet and the film formed from said surface coating greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with .each other, said adhesion being tenacious even when the composite film is subjected to contact with water over a long period of time.

10. The method of making a sheet or film suit-- able for use as a wrapping material which comprises forming a thin layer of sulfur on a base sheet of non-fibrous, transparent, cellulosic material, said layer being so minute as to have sub- I stantially no effect on'the transparency of the 6. The method of making a sheet or film suit sheet, and subsequently applying to said basev able for use as a wrapping material which com-*sheet a surface coating composed of a film-formprises forming a thin layer'of sulfur on a base ing base, a waxy material, a blending a ent and 11. A new article of manufacture comprising a non-fibrous transparent base sheet of cellulosic material, a surface coating on said base, and an anchoring medium comprising a thin layer of sulfur between said base and coating, said layer being so minute as to have substantially no effect on the transparency of the sheet, said sheet and said surface coating .exhibiting an adhesion greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with each' other, said adhesion being tenacious even when the composite film is subjected to contact with water over a long period of time. 1 I

12. A new article of manufacture comprising a base sheet of regenerated cellulose, a surface coating on said base sheet, and an anchoring medium comprising a thin substantially transparent layer of sulfur between said base sheet and coating, said sheet and said surface coating exhibiting an adhesion greatly improved over that exhibited between a similar sheetand surface coating when in direct contact with each other, said adhesion. being ten'acious even when the composite film is subjected to contact with water over a long periodof time.

' 13. A new article of manufacture comprising a base sheet of regenerated cellulose, a moistureproof surface coating on said base sheet, and an anchoring medium comprising a thin substantially transparent layer of sulfur between said base sheet and coating, said sheet and said surface coating exhibiting an adhesion greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with each other, said adhesion being tenacious even when the composite film is subjected to contact with water over a long period of time.

14. A new article of manufacture comprising a base sheet of regenerated cellulose, a surface coating comprised of a film-forming composition on said base sheet, and an anchoring medium.

comprising a thin substantially transparent layer of sulfur between said base sheet and coating, said sheet and said surface coating exhibiting an adhesion greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with each other, said adhesion being tenacious even when the composite film is subjected to contact with water over a long period of time.

15. A new article of manufacture comprising a base .sheet of regenerated cellulose, a surface coating comprised of a film-forming base and a waxy material on said base sheet and an anchor ing medium comprising a thin substantially transparent layer of sulfur between said base sheet and coating, said sheet and said surface coating exhibiting an adhesion greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with each other, said adhesion being tenacious even when the composite film is subjected to contact with water over a long period of time.

16. A new article of manufacture comprising a base sheet of regenerated cellulose, a surface coating comprised of a film-forming base, a waxy material and a blending agent on said base sheet, and an anchoring medium comprising a thin substantially transparent layer of sulfur between said base sheet and coating, said sheet and said surface coating exhibiting an adhesion greatly improved over that exhibited between a similar sheet and surface coating when in direct contact with each other, said adhesion being tenacious even when the composite film is subjected to contact with water over a long period of time.

17. A new article of manufacture comprising a base sheet of a regenerated cellulose, a surface coating comprised of a film-forming base, a waxy material, a blending agent and a plasticizer on said base sheet, and an anchoring medium comprising athin substantially transparent layer of sulfur between said base sheet and coating, said sheet and said surface coating exhibiting an ad hesion greatly improved over that exhibited between a similar sheet and surface coating when in direct contact. with each other, said adhesion being tenacious even when the composite film is subjected to contact with water over along period of time.

DOROTHY BA'I'EMAN MANEY. 

